Such forming apparatus is described in patent FR-2 028 281 and comprises: firstly, beneath the horizontal path of a thermoplastic web, a mold block vertically movable between a high position for thermoforming receptacles, in which position said mold block is pressed against said web, and a low position for unmolding said receptacles, the mold block being provided with at least one forming chamber open to the top thereof and having the same inside shape as the article or receptacle to be thermoformed; and secondly, above said horizontal path, a counter-mold block which may be fixed or free to move a little vertically between a high position in which the bottom end of the counter-mold block is raised a little from the horizontal path of the thermoplastic web, and a low position in which said bottom end is in contact with said thermoplastic web and urges it against the top end of the mold block when in its high position, i.e. against the top edge of each of the forming chambers. The counter-mold block is provided with as many cylindrical cavities as the mold block has forming chambers, with each chamber being axially aligned with a corresponding one of said cavities.
In that type of prior apparatus, a forming punch is mounted to move vertically inside each cylindrical cavity with a degree of lateral clearance relative to the inside face of the cavity and coaxially relative both to said cavity and to the corresponding forming chamber. The forming punch has an inactive top portion and an active bottom portion, with the inactive portion of the punch remaining inside the cylindrical cavity while its active portion is suitable for penetrating into the corresponding forming chamber down to the vicinity of the bottom thereof, and has dimensions that are similar to but smaller than the dimensions of the forming chamber, with the annular space delimited between the peripheral face of the forming punch and the inside faces of both the forming chamber and the cylindrical cavity being periodically connected to a source of compressed gas.
In that type of prior forming apparatus, the active portion of the forming punch is surmounted by a cylindrical inactive portion of a diameter identical to that of the top end of said active portion, and of height that is not less than the vertical stroke of said active portion. As a result, considerable mass needs to be accelerated and decelerated at the beginning and at the end of each downstroke and of upstroke of the forming punch, and this prevents the number of strokes that the punch can perform per unit time being increased. Another drawback of that prior apparatus lies in the fact that the inactive portion of the forming punch is very long, i.e. its height is not less than the height of the active portion of said punch, and the volume of the annular space around the inactive portion of the punch constitutes a considerable dead volume in which large quantities of compressed gas are needlessly consumed. In addition, the annular space surrounding the forming punch and delimited by the punch and the inside face of the cylindrical cavity of the counter-mold block cannot be hermetically sealed at the end of the punch downstroke, and as a result the compressed gas for pressing a determined zone of the thermoplastic web against the inside face of the forming chamber can only be injected into said annular space when the punch stops at the end of its downstroke.
An object of the present invention is to eliminate or at least to mitigate the above-mentioned drawbacks.